Insulation resistance tester

ABSTRACT

An insulation resistance tester is provided having a logarithmic conversion circuit. A simple circuit arrangement is employed making use of a logarithmic relation between forward voltage and current of a semiconductor element, such as a diode, or a logarithmic relation between the voltage and current between the base and emitter of a transistor.

BACKGROUND OF THE INVENTION

The present invention relates to an insulation resistance tester having a meter with a logarithmic scale for indicating resistance values.

Various circuits have been used for logarithmic conversion in the insulation resistance tester, such as a circuit using the Zener characteristic of Zener diodes, a circuit using an approximating polygonal line provided by diodes, and so forth. The first-mentioned circuit, however, requires specially selected Zener diodes, while the second-mentioned circuit requires a large number of parts.

SUMMARY

Accordingly, the present invention provides an insulation resistance tester having a simple logarithmic conversion circuit using the fact that the voltage and current in the forward direction of a silicon diode have a logarithmic relationship, or that there is a logarithmic relationship between the base-emitter voltage and current of a transistor.

BRIEF DESCRIPTION OF THE DRAWING

The invention, together with further objects, advantages and features thereof, will be more clearly understood from the following description taken in conjunction with accompanying drawings.

FIG. 1 is an illustrative example of a scale on the meter of an insulation resistance tester.

FIG. 2 is a chart showing the relationship between the current I_(e) flowing through a measured resistance Rx and the current Im shown on the meter.

FIG. 3 is an illustrative embodiment of the invention.

FIG. 4 is an illustration of the relation represented by a later-mentioned equation (5) or equation (15).

FIGS. 5 to 7 are electrical schematic illustrations of different embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 1 showing an example of the scale marked on the meter of an insulation resistance tester to which this invention pertains, reference numeral 1 denotes a pointer or hand of the meter, adapted to point to infinity (∞) when no electric current is flowing through the meter. Section (a) of FIG. 1 corresponding almost to one third of the overall length of the scale has a graduation corresponding to the inverse number of the measured resistance Rx. Section (b) has a graduation corresponding to the logarithm of the measured resistance Rx. In the present specification, the term "logarithmic conversion circuit" is used to mean a circuit for realizing the graduations of the sections (a) and (b) in combination, i.e. including section (a) corresponding to the inverse number of the measured resistance Rx.

FIG. 2 shows the relationship between the electric current I_(l) flowing through the measured resistance Rx and the current Im shown on the meter. More specifically, FIG. 2 shows, for example, that the hand of the meter is swung fully to point zero of the meter as shown in FIG. 1 when the current I_(l) flowing in the measured resistance Rx is 1 mA, while the hand is not moved and points to infinity (∞) when the current I_(l) is not greater than 1 μA. FIG. 2 further shows that the current Im shown by the meter is proportional to the current I_(l) when the latter falls within the range of between 1 and 10 μA, whereas the current Im shown on the meter is in proportion to the logarithm when the electric current I_(l) exceeds 10 μA.

FIG. 3 schematically shows the circuit arrangement of a first embodiment of the invention in which E represents a high-voltage power source while D₁ and D₂ represent diodes which are thermally balanced by each other. Symbols r_(s) and r_(l) denote resistances, while M represents a meter having an internal resistance r_(m). Further, reference numerals 1 and 2 designate measurement terminals to which the resistance Rx to be measured is connected. A series circuit consisting of the high-voltage power source E, diode D₁ and the resistance r_(l) is connected between the measurement terminals 1 and 2. Also, a series circuit consisting of the meter M, resistance r_(s) and the diode D₂ is connected to both sides of the diode D₁.

The insulation resistance tester of the invention having the circuit arrangement shown in FIG. 3 operates in the following manner. The resistance Rx to be measured is connected between the measurement terminals 1 and 2. The electric current supplied from the high-voltage power source E to the measured resistance R_(x) is represented by I_(l) while the electric current supplied to the meter M is represented by I_(m).

Representing the forward voltage across a diode by Vd and the forward current by Id, the following well known relationship is established.

    I.sub.d =Is·{exp(q·Vd/k·T)-1}   (1)

where,

Is: backward saturation current

q: charge of electron

k: Boltzmann's constant

T: absolute temperature

Equation (1) can be transformed into equation (2) as follows:

    Vd=(k·T/q){l.sub.n (Is+I.sub.d)-l.sub.n IS}       (2)

On the other hand, equation 3 representing the voltages across the diodes D₁ and D₂ shown in FIG. 3 by V_(D1) and V_(D2), respectively is as follows:

    V.sub.D1 =V.sub.D2 +(r.sub.s +r.sub.m)·Im         (3)

Equation (4) is derived from the equations (2) and (3) above as follows:

    Im=1/(r.sub.s +r.sub.m)·(k·T/q){l.sub.n (Is+I.sub.l -Im)-l.sub.n (Im+Is)}                                     (4)

When the electric current Im in the meter is large, i.e. in the selection (b) as shown in FIG. 1, a relationship expressed by Is<Im<I_(l) is established partly because of the fact that the electric current flowing in the diodes D₁ and D₂ is large and partly because of the presence of the resistances r_(s) and r_(m). Therefore, equation (4) is transformed into equation (5) as follows:

    (k·T((r.sub.s +r.sub.m)·q))·l.sub.n I.sub.l =Im+(k·T/((r.sub.s +r.sub.m).sub.q))l.sub.n Im   (5)

The right side of equation (5) is represented by Y, various values of which are illustrated in FIG. 4. Namely, line (i) represents the relationship Y=Im. Similarly, a curve (ii) shows a relationship Y=(k·T/((r_(s) +r_(m))·q))l_(n) ·Im, while a broken line curve (iii) represents the relationship Y=Im+(k·T/((r_(s) +r_(m))q))·l_(n) I_(m).

The line and curves in FIG. 4 are drawn in the large region of the current Im on the meter and on the assumption that Is<<Im<<I_(l). Therefore, the section of the graph shown in FIG. 4 marked (b) (corresponds to section (b) of FIG. 1) correctly represents the magnitude corresponding to the current Im in the meter. On the other hand, the section marked (a) in FIG. 4 does not correctly represent the current Im in the meter, because in this section the current Im is small and does not meet the above-mentioned assumption.

As will be understood from the above equation, the following equation (6) is established in section (b) of FIG. 4, because the line (i) is dominative in this section.

    Im≈(k·T/((r.sub.s +r.sub.m)·q))·l.sub.n ·I.sub.l                                         (6)

From equation (6) above, it will be seen that, when the current Im is large, i.e. in section (b) of FIG. 1, the current Im in the meter is proportional to the logarithm of the current I_(l) flowing in the resistance Rx to be measured.

When the current Im in the meter is small, (r_(s) +r_(m))·Im takes a small value so that equation (7) is derived from equation (4) as follows:

    (Is+I.sub.l -Im)/(Im+Is)≈1                         (7)

Also, equation (8) is derived from equation (7) above as follows:

    Im≈I.sub.l /2                                      (8)

Thus, when the current Im in the meter is small, i.e. in section (a) shown in FIG. 1, the current Im in the meter is proportional to the current I_(l) flowing through the resistance Rx to be measured.

Therefore, in section (a) of FIG. 1, the resistance Rx is large, and it is possible to represent the current I_(l) by I_(l) =E/Rx. Thus, section (a) in FIG. 1 can have a graduation corresponding to the inverse number of the resistance Rx to be measured, as represented by the following equation (9).

    Im≈E/(2·Rx)                               (9)

In section (b) shown in FIG. 1, the value of the resistance Rx to be measured is comparatively small, so that the resistance r₁ cannot be neglected. Thus, the current I_(l) is represented by I_(l) =E/(Rx+r₁). Therefore, the relationship represented by the following equation (10) is established concerning section (b) in FIG. 1.

    Im≈(k T/((r.sub.s +r.sub.m)·q)){l.sub.n ·E-l.sub.n (Rx+r.sub.1)}                         (10)

Namely, in section (b) in FIG. 1, the graduations approximate the logarithm of the resistance Rx to be measured.

In order to simplify the explanation of the invention, the circuit shown in FIG. 3 has been described on the assumption that each of the diodes D₁ and D₂ consists of only one diode. However, diodes D₁ and D₂ may be comprised of a plurality of diodes connected in series or parallel. By so doing, it is possible to adjust or change the sensitivity of the meter or to change the graduations of the meter in accordance with the use by suitably combining a plurality of diodes. For instance, if the diode D₂ is comprised of n diodes connected in parallel as shown in FIG. 5, equation (4) would include a constant term, while in equation (8), the right member is changed.

The diodes used in the insulation resistance tester of the invention are preferably balanced thermally. A better result will be obtained by using diodes formed on a common wafer.

FIG. 6 shows another embodiment of the invention in which symbols B and E represent a D.C. power source and a high-voltage D.C. power source, respectively. Q₁ and Q₂ represent transistors which are thermally balanced by each other. R₁ and R₂ represent resistances, while a symbol M represents a meter having an internal resistance r_(m). Reference numerals 1 and 2 designate measurement terminals to which the resistance R_(x) to be measured is connected. Terminal 2 of the measurement terminals is connected through the resistance R₁ to the collector and base of the transistor Q₁ and also to the base of the transistor Q₂. The emitter of the transistor Q₁ is connected to the measurement terminal 1 through the high-voltage power source E and also to the emitter of the transistor Q₂ through the resistance R₂. Further, the emitter of the transistor Q₁ is connected to the collector of the transistor Q₂ through a series circuit of the D.C. power source B and the meter M.

In the embodiment of FIG. 6, the resistance Rx to be measured is connected between the measurement terminals 1 and 2. Representing the electric current supplied from the high-voltage power source E to the measured resistance Rx by I_(l), the electric current flowing through the meter M by Im and the D.C. current amplification factor of the transistors Q₁ and Q₂ by h_(FE), the base current of the transistor Q₂ is represented by Im/h_(FE), while the base current of the transistor Q₁ is represented by 1/h_(FE) ·(I_(l) -Im/h_(FE)).

When the measured resistance Rx is comparatively small and the current I_(l) is large, i.e. section (b) in FIG. 1, the base current of the transistor Q₁ can be given by I_(l) /h_(FE). Then the relationship represented by the following equation (11) is established, in which the voltage between the base and emitter of the transistor Q₁ is represented by V_(BE1).

    I.sub.l /h.sub.FE =Is·{exp(q·V.sub.BE1 /k·T)-1}(11)

Since the backward saturation current Is between the base and emitter is extremely small as compared with the base current I_(l) /h_(FE), the equation (11) is transformed into equation (12) as follows:

    V.sub.BE1 =k·T/q·l.sub.n (I.sub.l /Is·h.sub.FE) (12)

The transistor Q₁ is thermally balanced by the transistor Q₂. Representing the absolute temperature of the transistors Q₁ and Q₂ by T, the voltage V_(BE2) between the base and emitter of the transistor Q₂ is expressed by equation (13) as follows:

    V.sub.BE2 =(k·T/q)·l.sub.n (Im/Is·h.sub.FE) (13)

Also, the relationship expressed by equation (14) exists in the circuit shown in FIG. 6 as follows:

    V.sub.BE1 =V.sub.BE2 +R.sub.2 ·Im(1+1/h.sub.FE)   (14)

Assuming the D.C. current amplification factor h_(FE) is sufficiently large to satisfy the relation 1/h_(FE) <<1, equation (15) is derived from the preceding equations (12), (13) and (14) as follows:

    1/R.sub.2 ·(k·T/q)·l.sub.n I.sub.l =Im+1/R.sub.2 ·(k·T/q)·l.sub.n ·Im  (15)

The equation (15) corresponds to the equation (5) mentioned in the explanation of the first embodiment. Namely, representing the right member of the equation (15) by Y, the line (i) of FIG. 4 shows the relationship Y=Im. Similarly, the curve (ii) and the broken line curve curve (iii) represent the relationships Y=1/R₂ ·(k·T/q)·l_(n) Im and Y=Im+1/R₂ ·(k·T/q)·l_(n) ·Im, respectively.

Since the line (i) is dominant in section (b) of FIG. 4 as in the preceding embodiment, equation (16) is established as follows:

    Im≈1/R.sub.2 ·(k·T/q)·l.sub.n ·I.sub.l                                         (16)

Namely, when the current Im flowing through the meter is large, the current Im is proportional to the logarithm of the electric current flowing in the measured resistance Rx.

When the current Im flowing through the meter is small, the voltage V_(BE1) between the base and emitter of the transistor Q₁ can be expressed by the following equation (17), because the base current of the transistor Q₁ is given by 1/h_(FE) ·(I_(l) -Im/h_(FE)):

    V.sub.BE1 =(k·T/q)·l.sub.n 1/Is·1/h.sub.FE (I.sub.l -Im/h.sub.FE                                     (17)

In addition, the second term of right member of the equation (14) can be neglected because the current Im is small, so that equation (18) is derived as follows:

    V.sub.BE1 ≈V.sub.BE2                               (18)

Further, the following equation (19) is derived from the equations (13), (17), (18) and (19), because the D.C. current amplification factor h_(FE) is expressed by 1/h_(FE) <<1.

    Im≈I.sub.l                                         (19)

Also, the sensitivity of the meter M can be selected as desired by adding a shunting resistance (not shown) to the meter M. As has been described, when the current Im flowing through the meter is small, i.e. in section (a) in FIG. 1, the current Im is proportional to the current I_(l) flowing through the measured resistance R_(x).

Therefore, in section (a) in FIG. 1, the current Im is expressed by I_(l) =E/R_(x), because the measured resistance Rx is very large, so that the graduations in section (a) in FIG. 1 are marked corresponding to the inverse number of the measured resistance Rx, as in the case of the circuit shown in FIG. 3.

    Im≈E/R.sub.x                                       (20)

In section (b) of FIG. 1, the resistance R₁ cannot be neglected because the measured resistance Rx is comparatively small, so that the current I_(l) is expressed by I_(l) =E/(R_(x) +R₁). Therefore, a relationship given by the following equation (21) is established in section (b) in FIG. 1.

    Im≈1/R.sub.2 ·(k·T/q)·{l.sub.n E-l.sub.n (R.sub.x +R.sub.1)}                                       (21)

Thus, in section (b) in FIG. 1, the graduations are formed approximately in proportion to the logarithm of the measured resistance R_(x), as in the case of the circuit shown in FIG. 3.

In order to simplify the explanation of the invention, transistors Q₁ and Q₂ have been described as comprised by only one transistor. Needless to say, however, it is clear that transistors in a Darlington connection can be used for each of these transistors. Thus, in the present specification, Q₁ and Q₂ are used to designate not only single transistors but also a plurality of transistors using a Darlington connection.

As in the case of the circuit shown in FIG. 3, transistors Q₁ and Q₂ of this embodiment are preferably balanced thermally. Again a better result will be obtained if the transistors are formed on a common wafer.

In order to facilitate the understanding of the invention, an assumption has been made that transistors Q₁ and Q₂ have equal D.C. current amplification factors. The coincidence of the amplification factor, however, is not essential. Namely, the amplification factors can be treated as being constant, if there is any constant relationship between the amplification factors of transistors Q₁ and Q₂.

Although in FIG. 6 the meter M is connected to the collector of the transistor Q₂, this is not exclusive and an equivalent effect is obtained by connecting the meter M to the emitter circuit of the transistor Q₂ as shown in FIG. 7.

In general, meter M involves a linearity error. This error has been conventionally corrected by means of a plurality of variable resistances incorporated in polygonal line circuits. However, according to the invention, the linearity error can be corrected easily by making the current Im in the meter variable. This can be achieved by using a variable resistance as the resistance r_(s) in the embodiment shown in FIGS. 3 and 5, and by using a variable resistance as the resistance R₂ in the embodiment shown in FIGS. 6 and 7.

As has been described, the present invention offers a great advantage that the logarithmic conversion in the insulation resistance tester can be achieved by means of a comparatively simple arrangement employing two diodes, resistance and a meter, or by a comparatively simple circuit incorporating two transistors, resistance and a meter. 

We claim:
 1. An insulation resistance tester comprising first and second semiconductor elements thermally balanced by each other, each of said semiconductor elements having a logarithmic characteristic, a series circuit including said second semiconductor element, a resistance and a meter, said first semiconductor element being connected in parallel with said series circuit.
 2. An insulation resistance tester as claimed in claim 1, wherein said first and second semiconductor elements are diodes.
 3. An insulation resistance tester having a logarithmic conversion circuit comprising a first and a second transistor thermally balanced by each other, said first and second transistors having D.C. current amplification factors which have a fixed relationship with respect to each other, said logarithmic conversion circuit having a meter therein coupled to said first and second transistors for making use of the relationships between the current and voltage between the base and the emitter of each of said first and second transistors.
 4. An insulation resistance tester as claimed in claim 3, wherein the base and collector of said first transistor and the base of said second transistor are interconnected, said emitter of said first transistor being connected to the emitter of said second transistor through a resistance, said logarithmic conversion circuit having a serially connected power source and a meter connected to the collector of said second transistor.
 5. An insulation resistance tester as claimed in claim 3, wherein the base and the collector of said first transistor and the base of said second transistor are interconnected, said emitter of said first transistor being connected to the emitter of said second transistor through a series circuit of a resistance and a meter, said logarithmic conversion circuit connected to the collector of said second transistor through a power source. 